1. Field of the Invention
The invention relates to a process and to an apparatus for dosing and applying liquid or pasty media to an object.
2. Description of the Relevant Art
Surface Mount Device (SMD) technology has been increasingly used of late for the construction of circuit boards. In this technology the SMD devices are directly soldered to the contact areas or lands on the circuit board; soldering paste being applied to the lands prior to insertion. This, for example, takes place in screen printing using a screen having the pattern of the lands. The soldering paste is applied to the screen where it passes through the screen openings and on to the circuit board where the devices are subsequently fixed to the soldering paste. In another process the devices are fixed by an adhesive to the circuit board and are then soldered in a wave (solder) bath. The adhesive points are applied with a dosing device or dispenser, which comprises an adhesive-filled cartridge, having a cannula and plunger, which is pneumatically or hydraulically operated. The adhesive quantity is controlled as a function of the pressure and the time.
The disadvantage of applying the soldering paste by screen process printing is that it is very complicated and costs are high. It is necessary to produce large batches of circuit boards, which have to be stored, and the screens must be separately produced and must be cleaned and checked after use.
Due to the disadvantages of the screen process, a process for applying the soldering paste by means of a dispenser, in which the latter is moved over a positioning table to the desired point and pressure surges are applied to the dispenser plunger, is often used. In the case of known dimensions of the dispenser and constant pressure, it is possible to dose the discharge quantity by means of the duration of the pressure surges. However, it has been found that the dosing is not accurate enough, because of the ever larger air space in the dispenser which causes the pressure surges to be damped through the displacement of the plunger and consequently the application quantity is influenced.
The above shortcomings are addressed by the design of the present invention.